Fuel cells generally consist of a polymer electrolyte membrane which selectively transports protons and of a pair of catalyst electrodes (fuel electrode and air electrode) between which the polymer electrolyte membrane is held. Fuel cells with this configuration are capable of continuous electric energy production by using a fuel gas, a gas containing hydrogen, supplied to the fuel electrode as well as an oxidizing gas, a gas containing oxygen, supplied to the air electrode.
The polymer electrolyte membrane consists of electrolyte which includes a polymer ion exchange membrane such as sulfonic group-containing fluorine resin ion exchange membrane or hydrocarbon resin ion exchange membrane. The polymer electrolyte member needs to contain a given amount of water for ion transport capability.
The catalyst electrode includes a catalyst layer and a gas diffusion layer. The catalyst layer contacts the polymer electrolyte membrane and promotes redox reactions in the catalyst electrode. The gas diffusion layer is located on the opposite side of the catalyst layer from the polymer electrolyte membrane and exhibits air permeability and electric conductivity. Moreover, the gas diffusion layer includes a carbon coat layer and a gas diffusion base layer. The carbon coat layer contacts the catalyst layer for improving the contact reliability between the gas diffusion layer and catalyst layer. The gas diffusion base layer diffuses therein supplied gas for uniform gas transport to the catalyst layer. The catalyst layer of the fuel electrode contains, for example, platinum or platinum-ruthenium alloy. The catalyst layer of the air electrode contains, for example, platinum or platinum-cobalt alloy. An assembled stack of a polymer electrolyte membrane and a pair of catalyst electrodes each including a catalyst layer, a carbon coat layer and a gas diffusion base layer is referred to as a membrane electrode assembly (hereinafter “MEA”).
MEAs may be electrically connected in series by stacking them atop each other. Conductive separators are interposed between the MEAs to avoid possible mixing between fuel and oxidizing gases as well as to establish electrical connection between the MEAs.
The separators are of two types: a fuel electrode separator which contacts the fuel electrode; and an air electrode separator which contacts the air electrode. The fuel electrode separator includes fuel gas flow channels through which a fuel gas is supplied to the MEA, and the air electrode separator includes oxidizing gas flow channels through which an oxidizing gas is supplied to the MEA.
As described above, the polymer electrolyte membrane needs to contain a given amount of water in order to offer ion transport capability. Thus, the gas is generally pre-humidified to ensure sufficient water content in the fuel cell. However, a humidifier for reaction gases does not contribute to electricity generation. Moreover, it requires an installation space. Thus, successful development of fuel cells that require no humidifiers is expected to lead to downsized fuel cell systems for overall cost reduction.
Moreover, if fuel cells requiring no humidifier can be developed, energy loss by such a humidifier can be avoided and therefore overall electricity generation efficiency increases. There has therefore been a continuing need in the art for the development of fuel cells capable of maintaining electricity generation efficiency even when supplying less- or non-humidified reaction gases.
Methods are known by which water generated during the operation of the fuel cell is kept within the fuel cell to ensure a given amount of water in the fuel cell without having to humidify reaction gases (see, e.g., Patent Document 1).
The fuel cell disclosed by Patent Document 1 is so configured that the oxidizing gas supplied in the oxidizing gas flow channels and the fuel gas supplied in the fuel gas flow channels are made to flow in opposite directions, and a coolant flow channel is provided above the oxidizing gas outlet for partial cooling of the oxidizing gas outlet.
FIG. 1 is a cross-sectional view illustrating a fuel cell disclosed by Patent Document 1. The fuel cell includes MEA 1 and a pair of air electrode separator 2 and fuel electrode separator between which MEA 1 is held. Air electrode separator 2 includes oxidizing gas flow channels 8, and fuel electrode separator 3 includes fuel gas flow channels 16.
As shown in FIG. 1, oxidizing gas and fuel gas flow in opposite directions. Moreover, coolant flow channel 15 is provided above oxidizing gas outlet 10, thereby cooling the oxidizing gas flowing near gas outlet 10. Cooling the oxidizing gas flowing near gas outlet 10 causes condensation of moisture in the gas, whereby water in the oxidizing gas can be recovered. In this way water generated in the fuel cell can be retained in the fuel cell.
In the fuel cell disclosed by Patent Document 1, all of the oxidizing gas flow channels are made uniform in width and depth for uniform distribution of oxidizing gas in the fuel cell.
Technologies are presented by which gas flooding is avoided by appropriately increasing or decreasing the cross sectional areas of the reaction gas flow channels provided in the separators along their lengths (see, e.g., Patent Document 2).
Technologies are also known by which curling of metal separators, which are made of stamped metal plate and include multiple gas flow channels formed therein, is reduced by using different flow channel volumes for adjacent gas flow channels (see, e.g., Patent Document 3).
Technologies are also known by which the reaction gas distribution is made uniform in the fuel cell by controlling the respective volumes of gas flow channels in the separators (see, e.g., Patent Document 4).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-249243
[Patent Document 2] Japanese Patent Application Laid-Open No. 2006-114387
[Patent Document 3] Japanese Patent Application Laid-Open No. 2005-32578
[Patent Document 4] U.S. Patent Application Publication No. 2007/0105001